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3507 :: block expression

• woggioni (orginal) [2018-01-27T14:34:56+01:00] view original

  It would be really nice to be able to write something like this

  let a = block:
      echo "hello"
      1
  Run

  but this does not compile because block is a statement

  a trivial workaround is

  let a = (proc() : auto =
      echo "hello"
      1
  )()
  Run

  which is quite ugly (and reminds me Javascript, which makes it just uglier) then I tried to write a template to keep using a beautiful syntax

  template scope(body) =
      let f = proc() : auto =
          body
      f()
  let a = scope:
      echo "hello"
      1
  Run

  but it does not work, I also tried this:

  template scope(body) = (proc() : auto = body) ()
  let a = scope:
      echo "hello"
      1
  Run

  but it also fail to compile.. What is the correct way to do it?

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• woggioni (orginal) [2018-01-27T15:02:18+01:00] view original

  I'll answer my own question.. It seems that I just forgot to specify the template return type:

  template scope(body : typed) : auto = (proc() : auto = body) ()
  
  let a = scope:
      echo "hello"
      1
  Run

  this works fine. There are some things that are not very clear to me about metaprogramming, it would be nice to have a more extended documentation about it. For example I see that templates can not only return an untyped value (which is just a piece of AST), but also a concrete value (in this example an integer). Is this possible also for macros (I would say no, but maybe I miss something)?

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• woggioni (orginal) [2018-01-27T16:50:07+01:00] view original

  I also noticed another problem

  let a = (proc() : auto =
      let b = 4
      6
  )()
  echo b
  Run

  this clearly does not compiles since b is accessed from an outer scope. This one should be the equivalent written with templates:

  template bxpr(body : typed) : auto = (proc() : auto = body)()
  
  let a = bxpr:
      let b = 4
      6
  echo b
  Run

  but, surprisingly, this compiles and prints 4 on stdout. But what is really funny is that, during the compilation it prints

  
  test.nim(4, 9) Hint: 'b' is declared but not used [XDeclaredButNotUsed]
  test.nim(3, 5) Hint: 'a' is declared but not used [XDeclaredButNotUsed]
  Run

  XD

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• jcosborn (orginal) [2018-01-27T17:29:57+01:00] view original

  Here's a related thread: https://forum.nim-lang.org/t/2201

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• DeletedUser (orginal) [2018-01-27T19:36:14+01:00] view original

  Moral of the story, this works:

  let a = (block:
    echo "hello"
    1)
  Run

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• woggioni (orginal) [2018-01-27T20:56:35+01:00] view original

  @Hlaaftana Yes, I'd just like to omit parenthesis without losing the scope semantics

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• Udiknedormin (orginal) [2018-01-29T00:20:11+01:00] view original

  Well... the template is the right thing to do but without closure magic --- just use a block:

  template scope(code): auto =
    block:
      code
  
  let a = scope:
    echo "hello"
    1
  
  echo a
  Run

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• woggioni (orginal) [2018-01-29T08:00:49+01:00] view original

  @Udiknedormin Thanks, you code works! Actually I was using this:

  template scope(body : typed) : auto = (block: body)
  Run

  but I guess ther is something I miss about the semantics of typed because this code

  template scope(body : typed) : auto = (block: body)
  let c = scope:
      let b = 5
      1
  echo b
  Run

  prints 5. Without the type of the template parameter the scoping works instead. I read the documentation about typed vs untyped here but this remains quite obscure to me.

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• bluenote (orginal) [2018-01-31T16:09:42+01:00] view original

  There should be no need for such a template, this must be a regression.

  Since this PR blocks should be an expression (see corresponding issue). There is also a test case. However the test case indeed seems to fail now on devel, not sure if testament somehow ignores the test...

  Update: My compiler was just outdated. This should work on devel.

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